The present invention relates generally to stents which are implantable or deployable in a vascular or endoluminal location within the body of a patient to maintain the lumen open at that location, and more particularly to improvements in stent.
Stents are expandable prostheses employed to maintain narrow vascular and endoluminal ducts or tracts of the human body open and unoccluded, such as a portion of the lumen of a coronary artery after dilatation of the artery by balloon angioplasty. While vascular usage is frequently discussed in this application, it will be understood by those skilled in the art that stents having the characteristics and features of the present invention may be implanted in other ducts or tracts of the human body to keep the lumen open, such as in the cerebral circulation system, tracheo-bronchial system, the biliary hepatic system, the esophageal bowel system, and the urinary tract system
In the case of an occluded coronary artery, for example, the original blockage is typically attributable to fatty deposits or plaque on the inner lining of the vessel. A different mechanism, however, produces a new blockage after an angioplasty procedure is performed to compress the deposits against the inner lining of the vessel, as by use of balloon angioplasty, or to virtually entirely remove the deposits, as by use of laser angioplasty or rotational cutting. The blood vessel wall is subjected to trauma by any of these procedures, which results in hyperplasia of the neointima, i.e., a rapid proliferation of muscle cells in the affected region of the wall, to cause restenosis and re-occlusion of the vessel lumen in a significant percentage of angioplasty patients within a period of from three to six months following the initial procedure.
To avoid this re-occlusion and to maintain the lumen of the vessel open, it is customary procedure to install a stent at the site in the vessel where the angioplasty was performed. The stent is deployed by radial expansion under pressure exerted by the inflating balloon of a balloon catheter on which the stent is mounted, to engage the inner lining or surface of the vessel wall with sufficient resilience to allow some contraction but also to provide a degree of stiffness to resist the natural recoil of the vessel wall following expansion.
Trends and extensions of increased knowledge and methods in practical cardiology are based primarily on advances in basic science and applied technology. For example, ten years ago, treatment of myocardial infarction (GI) stressed limiting physical injury and damage and focused principally on rehabilitation. The treatment strategy for acute MI was followed by a period of use of a thrombolytic agent. New techniques, new catheters, new stents and guidewires and improved fluoroscopic x-ray machines have more recently enabled treatment of acute MI with interventional catheter techniques. In one of these techniques, involving an angioplasty procedure, a small guidewire is advanced through an occlusion of a coronary artery which is attributable primarily to a thrombus, a balloon catheter is then advanced along the guidewire, and the balloon is inflated at the site of the thrombus to open the lumen of the artery. A stent is deployed at the lesion site either concurrently with or immediately following the angioplasty procedure to provide the necessary mechanical support to hold the lumen of the dissected vessel wall open.
This technique has been applied very successfully in coronary vessels which have a range of diameters from approximately 2.5 to 3.5 millimeters (mm). However, present day successful treatment of vessels having diameters smaller than 2.5 mm remains quite limited, because currently available apparatus and stent delivery systems are inadequate to negotiate such small vessel sizes to allow installation therein.
A wide clinical spectrum of diseases exists that would be receptive to beneficial treatment of vessels smaller than 2.5 mm in diameter. One such instance is treatment of side branches of the coronary arteries, which has a beneficial indication. A capability to treat vessels other than coronary vessels but of similarly small diameter, such as vessels enabling blood circulation in the brain, would likewise be desirable.
Ischemic stroke is characterized by pathophysiological characteristics which are very similar to those of MI. An artery is occluded either by an embolized thrombus as in patients with atrial fibrillation, or by a local thrombus that builds up on an arteriosclerotic vessel wall. Often these arteriosclerotic vessels are undergoing a local dissection, which limits the blood flow and activates the coagulation system. Access to small occluded arteries of the brain or other parts of the body for implementing procedures to allow adequate blood flow therethrough is a highly desirable objective for treating millions of persons likely to suffer stroke each year.
One of the technical prerequisites for successful treatment in these respects is the availability of a stent, and related delivery system, which is sufficiently small and thin that it can navigate and be and deployed in these tiny vessels without occluding the lumen thereof It is also essential that the stent be highly visible during and after implantation to enable proper deployment and aftercare by the physician. The latter attribute is especially important for treatment of intracerebral arteries, because of the obstacle to x-rays presented by the skull which makes precise visualization of a small thin stent extremely difficult. The stent should, therefore, be sufficiently radiopaque for valization without need for its struts to be made so large that the stent itself creates an unacceptable obstruction of the lumen of the vessel.
Another prerequisite of a successful treatment of these extremely small diameter vessels is that the stent delivery system should be highly flexible to allow it to be advanced along the anatomy of the cerebral circulation.
In addition, the total stent delivery system must be of extremely small profile, which will allow vessels of 2.0 mm, 1.75 mm or even 1.50 mm diameter to be addressed. No currently available stent delivery system has a balloon with a diameter less than about 2.5 mm when inflated at nominal pressure.
Therefore, it is a principal aim of the present invention is to provide stents and stent delivery systems having such attributes and characteristics, so as to enable successful treatment of extremely small diameter blood vessels and other ducts, tracts or conduits of the human body, without unacceptable obstruction of the vessel lumen itself.